Electron discharge device



Sept. 16, 1941. sMlTH ETALY' 2,256,297

ELECTRON DISCHARGE DEVICE Filed Aug. 51, 1959 2 Sheets-Sheet 1 INVEN TOR. PHIL/P ISM/TH 1W0 L L0 YD P. GARNER A TTORN E Y.

p 1941. P. T. SMITH ET AL 2,256,297

ELECTRON DISCHARGE DEVICE Filed Au 51, 1939 2 Sheets- Sheet 2 INVEN TOR. PH/L/P Z'SM/ TH All/0 LLOYD P. GARNER ATTORNEY.

Patented Sept. 16, 1941 uurrsn star ELECTRON DISCHARGE DEVICE Application August 31, 1939, Serial No. 292,822

6 Claims.

Our invention relates to high powered electron discharge devices, particularly to cathodes for such devices.

Cathode-to-anode space current and power output of conventional radio tubes is limited by the heating of the grid wires caused by electron bombardment. If all electrons could be directed between the grid wires when traveling from the cathode to the anode, wire heating would be reduced and the space current density and power level of the tube could be materially increased.

An object of our invention is to make an emcient electron discharge device adapted to deliver large amounts of alternating current power with minimum grid wire heating.

Another object of our invention is a radio tube with an improved cathode adapted to emit space current to the anode of high density without overheating the grid wires.

One tube constructed according to our invention comprises a cathode, grid electrodes and anodes arranged as shown in our co-pending application, Serial No. 258,868 filed February 28, 1939, and assigned to the same assignee as the present invention, the electrodes preferably being planar with a cathode of the filamentary U-shaped type comprising a relatively wide ribbon of refractory metal supported with the faces of the two sections of the ribbon parallel to grid and anode electrodes. The surface of the filament has a number of longitudinal grooves or flutes, each groove being arcuate in cross section with the ridges between the grooves in alignment with the wires of the grid so that the curved faces of the grooves are in full view of the anode between the grid wires. The ridges between the grooves of the filament are so constructed according to our invention that no electrons are emitted from the ridges directly opposite the grid wires parallel to the ridges.

The characteristic features of our invention are defined with particularity in the appended claims and preferred embodiments of the invention are described in the following specification and shown in the accompanying drawings in which Figure l is a perspective view of the electrodes of one tube embodying our invention, Figure 2 is an enlarged end View of the electrodes of Figure 1, Figure 3 is an enlarged perspective view of a cathode embodying our invention, Figure 4 is a detailed view of a fluted cathode and Figures 5, 6, '7 and 8 are detailed sectional views of cathodes embodying our invention.

The tube chosen for illustration may comprise a metal envelope, as described in our co-pending application, supra, with lead-in conductors for supporting the electrodes. Extending centrally from one end of the envelope is the relatively wide ribbon-like filamentary cathode l, the ribbon being bent into two parallel sections 2, each As best shown in Figure 2, the wires of the,

control grids t and 5 are opposite the ridges of the cathode. In operation the electrostatic field distribution between the control grid Wires and at the curved surfaces of the cathode flutes is such that the electrons leave normal to the surface of the flutes and form beams which pass between the spaced conductors of the grid and terminate on the anode. The number of electrons striking the grid wires is substantially reduced and most of the energy of the space current is dissipated in the anode circuit. Where a high cathode temperature and high emission current is required the cathode preferably is an uncoated pure metal such as tantalum, tungsten, or molybdenum. Electrons leaving the cathode normal to the curved surface of the flutes entrain in the beams thus formed and pass between the grid wires.

While flutes may be easily pressed into sheet metal and the cross sectional configuration of flutes designed to form electron beams, it has been found that considerable current nevertheless flowed to the grid Wires opposite the ridges. Examination of the cathode showed that the dies for pressing the flutes would leave the ridges between flutes somewhat rounded, as shown in Figure 4, from the comparatively flat surfaces of which considerable emission current would flow to the grid wires.

One cathode, constructed according to our invention, comprises a plurality of separate strips of metal with concaved emitting surfaces and with means to effectively separate the emission of the several surfaces and confine the electrons to the beams between grid wires. According to one embodiment of our invention the metal of the ribbon along the ridges is removed so that the boundaries of each emitting surface terminates at the edges of the metal strips. As shown in Figure 5, slots out along the ridges as by a milling cutter, may be easily made with the sides 8 of the slots normal to the plane of the cathode. The slot may cut all the way through the sheet metal as shown, or may be out part way through to leave a narrow groove along the ridge. The edges 9 of the concaved emitting surfaces at the rim of the slots may be made knife-like in sharpness so that the boundary of the emitting surface is sharply defined. Substantially no electrons may leave the sides 8 of the slots because of the repelling force of the opposed side of the same slot. For strength, integral bridging pieces may be left by the milling cutter.

An alternative method of sharpening the edges of the emitting areas is illustrated in Figure 6. Several strips of metal, each the width of the emitting flutes, is die cast or pressed from sheet metal and the sides 8 of each strip is ground and honed to give the strip at the edge 9 of the emitting surface razor blade sharpness. The strips may then be placed side-by-side and the strips secured at their ends to terminal blocks connected with cathode lead-in conductors to hold the polished surfaces 8 of the strips together. Since the apexes of ridges between the flutes have negligible width all electrons from the concaved surfaces of the strips are directed into the electron beams.

Long narrow rectilinear members such as small metal ribbons, rods or strips at cathode potential disposed along the ridges of the cathode ribbon have been found to be most effective in forming electron beams and reducing grid current. Relatively wide slots ill are out from the ridges of the ribbon as shown in Figure 7, with connecting bridges l2 spaced along the slots. A metal rod or strip I3 is then joined as by spot welding to each bridge, the rod preferably being supported at a point near its center and held in position over and slightly above the slot. Alternatively, the metal along the ridges on either side of the bridges I2, may be depressed to form a narrow groove along the ridges, with sufiicient depth and curvature to clear the under side of rod l3 when the rods are welded in place. Where the cathode is comparativelylong as shown in Figure 3, several end-to-end rods or strips should be disposed along the cathode ridges. Since each rod contacts the cathode at only one point, none of the cathode heating current may pass through the rod, and the rods remain below emission temperature. The rods along the ridges function also to accentuate the beam forming characteristics of the emitting flutes. Good results have been obtained with a cathode of the type shown in Figure 3 in which eight flutes each about .110 inch wide in a tantalum ribbon 0.885 inch wide, .010 inch thick and 8.0 inches long between terminals. At an emission temperature of about 2200 C. the 16 electron beams on the two sides of the U-shaped cathode delivered 22 amperes to 1500 volt anodes spaced .450 inch from the oathode, and grid wires .030 inch in diameter oppo site the rods on the cathode ridge and spaced .070 inch from the ridges receive less than 0.40 ampere of current.

Another cathode constructed according to our invention for preventing electron emission from the ridges between the flutes and shown in de tail in Figure 8 comprises a series of parallel side-by-side flutes 3 pressed in a ribbon of refractory metal with the dimensions, shape and cross sectional configuration of the flutes shown in Figures 5, 6 or 7. The ridges are prevented from rising to emission temperature by shunting the cathode heating current from the ridges. The cathode heating current flows from one end of the cathode to the other along parallel lines and current along the ridges is effectively interrupted by notching each ridge with a number of transverse cuts or grooves M. The grooves are conveniently made by passing the ribbon transversely of its longitudinal axis under a milling cutter, which isadjusted, preferably, to make cuts through the metal of the ribbon at the ridges. The ridges receive no direct heating from a cathode heating current and there is insuificient heat conducted from the grooves to the ridges to raise the ridges to emission temperature. While the cathodes shown are of the planar type it is of course within the scope of our invention to form side-by-side grooves in the siu'face of a tubular cathode.

Electron discharge devices constructed with cathodes according to our invention are adapted for high space currents with minimum grid wire heating. Our novel cathode is easy to make and efiicient in operation.

We claim:

1. A cathode for an electron discharge device comprising a metal cathode core, means for heating the surface of the core to emission temperature, the emitting surface of said cathode being divided into a plurality of side-by-side channels, and strips along the boundaries of the channels to prevent electron emission from the ridges. M

2. A cathode comprising parallel side-by-side strips of metal, each strip being arcuate in cross section with the concaved surfaces of the strips being electron emissive at elevated temperatures, the edges of the metal strips along the boundaries of the emitting surfaces being sharpened to accurately define the outer boundary of emission from each concave emitting surface.

3. A cathode comprising parallel and side-byside strips of metal, each strip being arcuate in transverse cross section, the concaved surfaces of the strips being electron emissive at elevated temperatures, and a metal rod supported on the cathode opposite the edges of adjacent strips.

4. A cathode comprising a ribbon of refractory metal electron emissive at elevated temperatures connected at each end to power supply terminals, the ribbon being fluted with side-byside longitudinal grooves extending from one terminal to the other, the ridges between the grooves having transverse cuts to interrupt current flow along the ridges.

5. A cathode comprising side-by-side strips of metal, the surfaces of the strips being electron emissive at elevated temperatures, narrow rectilinear metal members supported on said strips along and opposite the edges of adjacent strips, said members being non-emissive while said strips are at emissive temperature to suppress emission along said edges.

6. An electron discharge device comprising an anode and a cathode, said cathode having a plurality of distinct electron emitting surfaces, the emitting surfaces of the cathode comprising the faces of side-by-side strips of metal parallel with the electron collecting surface of said anode, and means to suppress electron emission from the boundaries between adjacent emitting surfaces and to direct substantially all electrons from one surface into a beam independent of electrons from adjacent beams, said means comprising a narrow ribbon of metal supported on said strips of metal opposite each of said boundaries, each ribbon being supported in poor heat conducting relation with said strip and being substantially non-emissive while said strips are at emitting temperature.

PHILIP T. SMITH.

LLOYD P. GARNER. 

